The present invention relates to a switching circuit employing a photon coupling between a control portion and a load current carrying portion. The present invention may be used as electronic switches substituting for crossbar switches of conventional electromechanical relays in a communication system.
In order to acquire electronic switching circuits which substitute for conventional electromechanical relays, great efforts have been exerted. For example, U.S. Pat. No. 3,816,763 discloses solid-state relays which exploit photon coupling. Each of the solid-state relays has a control portion and a load current carrying portion. The control portion includes a light emitting diode (LED) and a current controller which controls current supplied to a photon generator, that is LED. In the load current carrying portion, a diode bridge for rectifying an alternating current produces direct current between positive and negative DC conductors. A photon-activated switching means, for example a light-activated silicon controlled rectifier (LASCR) switches the rectified load current between the DC conductors. Triggering energy for the LASCR is supplied by the photons radiating from the LASCR.
The switching sensitivity of the LASCR is controlled by a means for controlling the gate electrode current flow, which includes a bias signal-responsive variable impedance device, for example an NPN transistor. The NPN transistor is connected in a series circuit relation with the gate electrode of the LASCR by the connection of NPN transistor's collector electrode to the gate electrode. An emitter electrode connects to a cathode electrode of the LASCR. Bias signals are introduced through a base electrode of the transistor. To provide for switching at or near the point of zero instantaneous amplitude of the voltage between the load current terminals, a bias signal generating means, connected to the load current terminals, supplies a control signal to the means for controlling the gate electrode current. The bias signal generating means and the means for controlling the gate electrode current thereby vary the switching sensitivity of the photon-activated switching means to provide for the desired zero voltage switching.
When the instantaneous amplitude of the voltage present between the load current terminals is at or near zero, the bias signal generating means provides an inadequate bias signal to the means for controlling the gate electrode current, and it does not drain significant photon-generated current from the gate electrode. Under these conditions, the photon-activated switching means is the most sensitive for triggering by photons.
Such zero voltage switching photon coupled relay as mentioned above, however, has drawbacks as follows. When sufficient voltage appears between the load current terminals, the bias signal generating means provides an adequate signal for overcoming the threshold of operation of the means for controlling the gate electrode current, and any photon-generated current is effectively drained from the gate electrode. As a result, the photon-activated switching means then is rendered insensitive to triggering by the photons emitted from the photon generator. Further, the photons radiated from the photon generator impinge on the gate electrode of the LASCR, and simultaneously, erroneously impinge on the NPN transistor to bring it into the conductive state. In that case, even when the bias signal generating means is providing the inadequate bias signal, the transistor falls into the conductive state. Therefore, even in case where the instantaneous amplitude of the voltage between the load current terminals is at or near zero, the desired switching sensitivity is not attained. This is particularly true when the load current carrying portions of such relays are fabricated in the form of an integrated circuit (IC) on a single silicon chip.
In order to eliminate the disadvantage as in the latter, it has heretofore been executed to evaporate a metallic film on the NPN transistor so as to optically cut it off. In that case, however, such an NPN transistor requires a comparatively large area on an IC substrate. This leads to the disadvantage of conspicuously lowering the density of integration of such an IC.
In "SCR MANUAL" (furnished by G. E.), especially FIG. 4. 12 thereof, a silicon controlled rectifier (SCR) circuit which has a transistorized dynamic snubber to improve dv/dt withstand capability is disclosed. According to this circuit, in order to improve the dv/dt withstand capability of the SCR, a first transistor is disposed between a gate electrode and a cathode electrode of the SCR. A base electrode of the first transistor is connected to a cathode electrode of a second transistor, and also to an anode electrode of the SCR through a series connection of a capacitor and a resistor. The gate electrode of the SCR is connected to a collector electrode of the first transistor and a base electrode of the second transistor. The series connection of the capacitor and the resistor supplies current to the base electrode of the first transistor turning it "on" when anode voltage is rising.
The circuit as stated above, however, has the following disadvantages. Since such a circuit includes the capacitor therein, it is unsuitable for an IC. In addition, this circuit is unsuitable for a switching circuit which controls a relatively high power switched current output with a relatively low power control signal input, because the control portion and the load current carrying portion thereof are not electrically isolated perfectly.
Other relevant prior arts are as follows:
(1) U.S. Pat. No. 3,723,769 "Solid State Relay Circuit with Optical Isolation and Zero-cross Firing" (patented Mar. 27, 1973);
(2) U.S. Pat. No. 3,504,131 "Switching Network" (patented Mar. 31, 1970); and
(3) U.S. Pat. No. 3,413,480 "Electro-optical Transistor Switching Device" (patented Nov. 26, 1963).